1. Field of the Invention
The present invention relates to ceramics for forming a variety kinds of wiring boards such as packages for accommodating semiconductor device and multi-layer wiring boards. Particularly, the invention relates to ceramics for wiring boards produced by the co-firing with a low-resistance conductor such as copper, gold, silver or the like metal and to a method of producing the same.
2. Description of the Prior Art
The most widely used ceramic wiring board can be represented by the one having a wiring layer of a high-melting metal such as tungsten (W) or molybdenum (Mo) formed on the surface or in the inside of an insulating substrate of an aluminous sintered product.
In recent years, however, the frequency band of signals in which the wiring boards are used is shifting toward the ever high frequency region accompanying the a trend toward the highly sophisticated information technology. In the wiring boards which must transmit signals of such high frequencies, it is required to decrease the resistance of the conductor forming the wiring layer and to decrease the dielectric loss of the insulating substrate in a high-frequency region, in order to transmit high-frequency signals without loss.
However, the wiring layer formed on the above-mentioned conventional ceramic wiring board, i.e., the wiring layer formed of a high-melting metal such as W or Mo, has a large electric resistance and permits signals to propagate at a low speed. With such a wiring layer, furthermore, it is difficult to transmit high-frequency signals of not lower than 1 GHz. Therefore, it becomes necessary to form a wiring layer using a low-resistance metal such as copper, silver or gold instead of using W or Mo. However, these low-resistance metals have such low melting points that the wiring layer is not formed by the co-firing with alumina.
Recently, therefore, there has been developed a wiring board using an insulating substrate made of so-called glass ceramics comprising a composite material of a glass and ceramics. The glass ceramics has a dielectric constant of as low as about 3 to 7, and is not only suited for transmitting high-frequency signals compared to alumina ceramics but is also obtained by the firing at a temperature as low as from 800 to 1000.degree. C., lending itself well for being co-fired with a low-resistance metal such as copper, gold, silver or the like metal.
For example, Japanese Examined Patent Publication (Kokoku) No. 12639/1992 proposes a wiring substrate obtained by forming a wiring pattern of a low-resistance metal such as copper, silver or gold on a green sheet formed of a glass and an SiO.sub.2 filler, and co-firing the green sheet and the wiring pattern at 900 to 1000.degree. C.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 240135/1985 proposes a wiring board obtained by co-firing a green sheet comprising a filler such as Al.sub.2 O.sub.3, zirconia or mullite and a zinc borosilicate glass together with a wiring pattern of a low-resistance metal.
Japanese Unexamined Patent Publication (Kokai) No. 298919/1993 discloses glass ceramics in which mullite or cordierite is precipitated as a crystal phase.
The above-mentioned conventional glass ceramics can be formed by the co-firing with a low-resistance metal, but have a defect in that they have small coefficients of thermal expansion (about 3 to 5 ppm/.degree. C.).
That is, the wiring board is used mounting a variety of electronic parts (e.g., a chip of GaAs, etc.) thereon or being mounted on a printed board such as a mother board formed of an organic resin. The mounting is executed by the so-called brazing which produces a thermal stress between the wiring board and the printed board. The thermal stress is also generated between the wiring board and the printed board by the thermal hysteresis of when the semiconductor device or the like is operated and stopped repetitively. Here, the chip such as of GaAs has a coefficient of thermal expansion of from 6 to 7.5 ppm/.degree. C. and the printed board has a coefficient of thermal expansion of from 12 to 15 ppm/.degree. C., which are greatly different from the coefficient of thermal expansion of the above-mentioned glass ceramics. In the wiring board equipped with an insulating substrate formed of the known glass ceramics, therefore, the mounting portion is peeled off or is cracked due to the thermal stress at the time of mounting or due to the thermal stress of when the semiconductor device is operated and stopped repetitively, that stems from a large difference in the coefficient of thermal expansion. Therefore, such a wiring board has a very low reliability for mounting and is not satisfactory from a practical point of view.
Furthermore, the conventional glass ceramics has a large dielectric loss in the high-frequency region. Therefore, the wiring board equipped with an insulating substrate of such glass ceramics exhibits poor high-frequency characteristics and cannot be used for high-frequency applications where high-frequency signals are handled such as microwaves and millimeter waves.